Diagnosis is a fundamental concept in medicine. A proper diagnosis is crucial, as it can lead to appropriate treatment and care. Two important areas in the diagnostic process for people with intellectual disabilities who are suspected of being psychotic are recognizing atypical symptoms and differentiating between challenging behaviour and psychotic symptoms. More about differential diagnostics will be covered in the next chapter. This chapter focuses on helpful tools in the diagnostic process, as well as the use of third-party information in addition to self-reports. The topics include clinical interviews, tools, and checklists: the Psychopathology in Autism Checklist (PAC), the Aberrant Behaviour Checklist (ABC), the MINI Plus Neuropsychiatric Interview, the Positive and Negative Syndrome Scale (PANSS), and the Diagnostic Manual—Intellectual Disability 2 (DM-ID 2).